1. Field of the Invention
The invention relates to a method for etching semiconductor structures and to an etching composition for use in such a method.
2. Description of Related Art
Under bump metallization is being increasingly used in preference to wire bonding, to connect a semiconductor device to a packaging substrate. This metallization is not limited to the outer perimeter of the device, but may also be deposited on the top metal level of the device at a final stage of wafer processing.
In a typical under bump metallization structure, a bump or pillar of solder is connected to an underlying contact pad, which is typically aluminum, via an under bump metallization (UBM) made up of two or more layers. For example a lower layer in contact with the contact pad may promote adhesion of the bump to the contact pad and also functions as a diffusion barrier to protect the underlying integrated circuit, whereas an upper layer in contact with the solder bump is formed from a material that is wettable by the solder so as to strengthen the bond between the bump and the contact pad.
One conventional UBM structure includes a lower layer comprising titanium and an upper layer comprising copper. When such a structure is etched to form its final shape, the underlying titanium layer is etched to a greater extent than the overlying copper layer, resulting in undercut. This undercut results in less contact area to the metallic contact pad, as well as stress in the bump.
Conventional etching techniques using dilute hydrofluoric acid (dHF) produce an undercut not less than about 1 μm, whereas benchmarks for future generations of technology call for an undercut of 0.5 μm or less. Alkaline etching chemistries and hydrogen peroxide blends are also unsatisfactory, owing to their slow etch rates and high temperature requirements.